In current fuel burner designs, energy efficiency is a major consideration. One obvious way to reduce fuel consumption by a burner is to use other than a standing (constantly burning) pilot flame for ignition. There are two major types of non-pilot ignition sources, spark igniters and hot surface igniters. Both of these can be used directly to light a furnace. However, there are certain problems that arise with either of these types of igniters when direct ignition is used since they tend to fail more frequently than does pilot ignition. If direct ignition does not occur promptly once it is attempted, then it is necessary to shut off the flow of fuel and purge the furnace before attempting another lighting to prevent ignition with an unsafe amount of fuel within the combustion chamber. Neither is it satisfactory to reduce flow of fuel during direct ignition since this usually increases the difficulty of lighting the burner. Controls to implement direct ignition of furnaces are relatively complex because of this problem, and hence become expensive.
Because of these problems, many furnaces, particularly larger ones, now use an intermittent pilot burner which is lit each time there is a demand for heat from the fuel burner. Once such an intermittent pilot burner has been lit successfully, then it is used to light the main burner. Intermittent pilot flames are now usually lit by a spark igniter which is activated after the pilot burner valve has been opened. The ignition control holds open the pilot burner valve for a set period of time, and if the pilot flame is not sensed by the end of that time period, locks the system out. Ignition failure for the pilot light requires manual resetting of an interlock before further ignition attempts can occur. The time period allowed for setting the pilot flame is short enough to prevent a dangerous escape of gas and long enough to assure that the pilot flame will usually set if there is any chance that it will.
As mentioned, it is the usual practice to light these intermittent pilots with a spark igniter. However, there is no reason that a hot surface igniter cannot be used for igniting an intermittent pilot. Such an igniter typically comprises a short piece of conductive silicon carbide or other conductive refractory material placed in proximity to the fuel jets and through which is passed a current of sufficient magnitude to cause it to be heated to a temperature of at least 1000.degree. C.
Even more important than energy efficiency is safety in the operation of fuel burners. If flame is not present whenever either the main or pilot valve is open, fuel can accumulate in the burner chamber, creating a potentially dangerous condition. Obviously, there is a much greater potential for harm if the main valve is open without a flame being present simply because of the much greater amount of fuel which the main valve passes. In general a pilot valve can be open for some time before a dangerous situation is created, and by timing out the pilot valve lighting procedure, little risk arises.
It has been standard practice to provide some sort of flame sensor so as to assure that flame is present whenever a fuel valve is open. The reason for sensing flame when the main valve is open, is manifest. The reason for sensing the presence of the pilot flame is as a necessary condition for opening the main valve, to thereby assure that ignition of the fuel flowing through the main valve will occur. Typically flame is sensed by either some kind of heat sensor, by the ability of the flame to rectify an AC current, or by a photoelectric device which senses the radiation generated by the flame.
There are a number of patent references which pertain to the technology of safe and efficient management of burners which are lit by intermittent pilots. U.S. Pat. No. 4,360,338 (Katchka) tests only the pilot flame to be present before opening the main valve. U.S. Pat. No. 4,689,006 (Gann) discloses a spark igniter for an intermittent pilot where a single pole double throw relay is controlled by the pilot flame sensor to switch power from the igniter to the main valve. U.S. Pat. No. 4,565,520 (Gann) discloses main burner ignition conditioned on pilot ignition, with a limited trial ignition period for pilot ignition.